The present invention relates to a material for scintillators. It more particularly applies to the construction of very fast detectors of high energy photons, i.e. X or gamma photons and to the construction of tomographs.
It is known that one of the most accurate means for detecting the passage time of a photon involves the use of a plastic scintillator associated with a fast photomultiplier. There are a number of commercially available plastic scintillators, e.g. those known under the names PILOT U or NE 111. During their deexcitation in accordance with an interaction with a gamma photon, all these plastic scintillators emit a light pulse having a time constant of approximately 1.5 ns and whose intensity, i.e. the total number of photons of said pulse is proportional to the energy of the incident .gamma. photon. A light output of approximately 3000 to 4000 photons/MeV is normally obtained with such scintillators.
The device diagrammatically shown in FIG. 1 can be used for evaluating the time or time measurement performances of plastic scintillators, i.e. their transit time measurement performances, as well as for comparing these scintillators with other scintillators.
This device comprises two facing detectors D.sub.1 and D.sub.2, respectively incorporating thin plastic scintillators 2a and 2b associated with fast photomultipliers 3a, 3b. The latter are electrically connected respectively to high voltage power supplies 4a, 4b, while both are electrically connected to electronic amplification, selection and counting means, known to the Expert, and which also makes it possible to measure the time difference between the detection time of a .gamma. photon by the first detector D.sub.1 and the detection time of another .gamma. photon by the second detector D.sub.2, said two .gamma. photons, each having an energy of 511 keV, resulting from the annihilation of an electron and a positron and being simultaneously emitted in opposite directions from a point 6 of an object 1 in which a positron-emitting substance has been incorporated.
The results obtained for these time differences fluctuate and have a Gaussian distribution which is characteristic of the type of scintillator used for a given measuring cascade (photomultipliers 3a, 3b and electronic means 5). This distribution is itself characterized by its total mid-height width or FWHM.
In the case of plastic scintillators with a diameter of 20 mm and a thickness of 2 mm, the FWHM is approximately 150 ps, with an energy threshold such that only events resulting from photoelectric effects are selected, and approximately 200 ps by taking 90% of the detected events and with an energy threshold equal to or above 100 keV.
The main disadvantage of a plastic scintillator is that the plastic material from which it is formed is on the one hand constituted by elements whose atomic numbers are low and on the other hand also has a low density (approximately 1.1 to 1.2 g/cm.sup.3), due to the fact that it is formed from light atoms. Thus, the main effect governing the interactions of the .gamma. photons with said plastic material is the Compton effect. Consequently, there is no simple relationship between the energy of an incident .gamma. photon and the energy yielded by the latter to the plastic scintillator, so that any spectrometry of the .gamma. radiation is then impossible. It is therefore necessary either to lose a large number of interactions by not taking account of the rare .gamma. photons interacting by the photoelectric effect, or to analyse all the photons which have interacted without being able to select them as a function of their energy.
Another very accurate prior art means for detecting the passage time of a photon and which is more particularly described in French Patent Application No. 79 02053 involves the use of a scintillator made from cesium fluoride CsF, said material containing on the one hand cesium atoms, said element having a high atomic number (55), whilst on the other hand having a high density (approximately 4.6 g/cm.sup.3). However, it is inferior to plastic in two respects, namely its fluorescence time constant is approximately 2.5 to 3 ns (instead of approximately 1.5 ns for plastic) and its scintillation response is approximately 1500 to 2000 photons/MeV (instead of 3000 to 4000 photons/MeV for plastic). However, cesium fluoride makes it possible to carry out spectrometry with an interesting energy level, because its resolution is approximately 30% with .gamma. photons of 511 keV. Moreover, by replacing plastic scintillators 2a and 2b of the device of FIG. 1 by cesium fluoride scintillators with the same geometry, it is possible to obtain a FWHM of approximately 200 to 300 ps, as a function of the cesium fluoride quality used.
Thus, the time performances of a plastic scintillator are better than those of a cesium fluoride scintillator. However, if account is taken of the fact that the stopping power of cesium fluoride is higher than that of plastic, it can be affirmed that in the prior art and based on an equal detection efficiency, cesium fluoride gives better time results than plastic.
However, compared with plastic, cesium fluoride has a disadvantage in that the material is very difficult to obtain as a result of its very high hygroscopy, which is well above that of e.g. thallium-doped sodium iodide NaI(Tl). This makes it necessary to very tightly encapsulate this material to permit its use for the production of scintillators. Therefore, cesium fluoride scintillators are very expensive and far from easy to use.